1. Field of the Invention
The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a strategy to control a split powertrain HEV while the vehicle travels in reverse and vehicle state-of-charge (SOC) is low.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another electric motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a xe2x80x9cpowersplitxe2x80x9d configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier gear. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV""s potential benefits.
One such area of HEV development is controlling a powersplit HEV while traveling in reverse. In the prior art, a reverse gear in a transmission is engaged when the vehicle operator moves a shift lever to the reverse, or xe2x80x9cRxe2x80x9d, position. In an HEV, a variety of powertrain configurations based on vehicle conditions can require new strategies to move the vehicle in reverse.
A strategy for moving an HEV in reverse is known in the prior art. See U.S. Pat. No. 5,847,469 to Tabata et al. Tabata et al. describes an HEV using a conventional transmission to power the vehicle""s wheels. The patent describes a system for the electric traction motor alone to reverse the direction of the vehicle without reversing the rotation of the motor so long as there is enough battery charge. Otherwise, the engine is started to assist the motor.
A conventional transmission with a reverse gear could be considered an inefficient and unnecessary complication and expense in a split powertrain HEV. Alternatively, the electric traction motor alone is used to propel the vehicle in reverse direction. When moving in reverse, the ring gear torque, resulting from engine output, goes against the vehicle moving in reverse. Thus, using the engine while the vehicle is in reverse is undesirable. Nevertheless, if battery state-of-charge (SOC) is low, the engine may need to run to power a generator to charge the battery and allow the motor to operate.
Unfortunately, no strategy is known to control a split powertrain HEV while the vehicle travels in reverse with the engine running because the battery state-of-charge (SOC) is low and the electric traction motor requires electricity produced by the generator for reverse motive power.
Accordingly, an object of the present invention is to provide a strategy to control a split powertrain hybrid electric vehicle (HEV) when the vehicle travels in reverse, vehicle state-of-charge (SOC) is low and the powertrain is configured to only use the motor while traveling in reverse.
The powersplit hybrid electric vehicle (HEV) powertrain of the present invention has an engine, a traction motor, a generator, an electric energy storage device for storing electric energy, the electric energy storage device connected to the traction motor to power the traction motor, and the electric energy storage device connected to the generator to receive energy generated by the generator. The powersplit HEV powertrain also has a power transmission device having at least one forward drive position to move the HEV in a forward direction and at least one reverse drive position to move the vehicle in a reverse direction. The power transmission device is connected to the engine, the traction motor, and the generator motor. In addition, the powersplit HEV powertrain has a driver operated drive position selector comprising a reverse drive mode, a vehicle system controller comprising a reverse drive mode controller activated when the drive position selector is in the reverse drive mode, wherein the reverse drive mode controller preventing the battery SOC from continuously falling while meeting driver demand.
The powertrain reverse drive mode controller can be configured to determine whether the engine and generator motor are running, calculate a benefit power from the engine if the engine and the generator motor are running, compare the benefit power with a first predetermined value, determine whether a driver torque request plus the generator torque is greater than a predetermined maximum motor torque if the benefit power is greater than or equal to the first predetermined value, calculate a new generator torque request if the determination of whether a driver torque request plus the generator torque is greater than a predetermined maximum motor torque, determine whether the new generator torque request is greater than or equal to a second predetermined value, calculate a new generator speed for the new generator torque request if the new generator torque request is greater than or equal to the second predetermined value, determine whether the new generator speed is less than or equal to a maximum generator speed, and determine a new motor torque request if the new generator speed is less than or equal to a maximum generator speed.
The controller can also be configured to add a stop engine command if the benefit power is less than the first predetermined value, the new generator torque request is less than the second predetermined value, or the new generator speed is greater than a maximum generator speed.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.